Appliances for mobile communication should support sending and receiving in different frequency bands. Since an antenna can usually have an optimum radiation characteristic only for one frequency band at the resonant frequency of the antenna, however, communication appliances having a plurality of, but at least two, antennas are customary. In the case of appliances having a plurality of antennas, interactions between the individual antennas are often unavoidable, however. When a first antenna is active, for example, the radiation emitted by the first antenna is coupled into the second antenna again. Such coupling of the two antennas is often undesirable.
In addition, antenna arrangements are known in which a first signal path, which is connected to a first antenna, and a second signal path, which is connected to a second antenna, are coupled by means of directional couplers to a third signal path each. FIG. 1 shows such an antenna arrangement, which is known in the prior art.
The first antenna Ant1 is used for sending and receiving signals from a high-frequency band. The second antenna Ant2 is designed for frequencies from a low-frequency band. In this case, the low-frequency band is defined in that it comprises frequencies which are lower than the frequencies of the high-frequency band. It is possible for the frequency range of the low-frequency band to adjoin the frequency range of the high-frequency band.
The antenna arrangement shown in FIG. 1 has a first signal path SP1, a second signal path SP2 and a third signal path SP3. The first signal path SP1 has a switch S1 which can be used to connect the first signal path SP1 to further signal paths SPHF1, SPHF2, SPHF3 which are connected to a transmission and reception circuit for frequencies from the high-frequency band and which have various filters for high-frequency signals. In addition, the first signal path SP1 can be connected by means of this switch S1 to a terminating resistor R1. The first signal path SP1 is also coupled by means of a dual-band directional coupler DRK to the third signal path SP3. The first signal path SP1 is connected to the first antenna Ant1.
A second signal path SP2 has a second switch S2 which can be used to connect the second signal path SP2 to further signal paths SPLF1, SPLF2, SPLF3 which are connected to a transmission and reception circuit for frequencies from the low-frequency band and which have various filters for frequency ranges from the low-frequency band. Furthermore, the second signal path SP2 can be connected by means of this second switch S2 to a terminating resistor R2. The dual-band directional coupler DRK couples the second signal path SP2 to the third signal path SP3. The second signal path SP2 is connected to the second antenna Ant2.
In the positions of the first switch S1 and the second switch S2 which are shown in FIG. 1, the first antenna Ant1 is connected by means of the first switch S1 to the terminating resistor R1 and the second antenna Ant2 is connected by means of the second switch S2 to the further signal path SPLF1, which is connected to a transmission and reception circuit for a particular frequency range from the low-frequency band. Accordingly, the first antenna Ant1 is terminated and the second antenna Ant2 is active.
The third signal path SP3 is coupled by means of the dual-band directional coupler DRK to the first and second signal paths SP1, SP2. The third signal path SP3 also has measuring devices ME_forward and ME_reflected. In the switch position of the switches S1, S2 which are shown in FIG. 1, a signal from the transmission device for low frequencies is coupled into the second signal path SP2 via the signal path SPLF1 and the second switch S2. A certain signal component is coupled from the second signal path SP2 into the third signal path SP3 by means of the dual-band directional coupler DRK. This signal component reaches the measuring device ME_forward. This measurement can be used to determine a gain factor for the antenna arrangement and the transmission device.
In the second signal path SP2, the signal component which has not been deflected into the third signal path SP3 by means of the dual-band directional coupler DRK now reaches the second antenna Ant2 and is emitted thereby. However, a certain signal component is also reflected by the second antenna Ant2. The reflected signal component now takes the second signal path SP2 in the reverse direction and is to some extent coupled into the third signal path SP3 by the dual-band directional coupler DRK. In the third signal path SP3, this signal component reaches the measuring device ME_reflected. In this way, a possible mismatch in the second antenna Ant2 can be determined.
In a converse switch position for the switches S1, S2, the second signal path SP2 is connected to the terminating resistor R2 and the first signal path SP1 is connected to one of the further signal paths SPHF1, SPHF2 or SPHF3. Accordingly, the first antenna Ant1 is then active and the second antenna Ant2 is terminated.
Again, the dual-band directional coupler DRK prompts part of the inbound signal to be coupled out of the first signal path SP1, to be coupled into the third signal path SP3 and thus to reach the measuring device ME_forward, which ascertains the gain factor for the antenna arrangement. In addition, a signal component reflected by the first antenna Ant1 is to some extent coupled by means of the dual-band directional coupler DRK into the third signal path SP3, where it reaches the measuring device ME_reflected, which determines the mismatch in the first antenna.
The third signal path SP3 also has damping elements DE1, DE2, DE3, DE4, DE5, DE6. These ensure that only a small signal component is coupled into the third signal path SP3 from the second or first signal path SP1, SP2. Customary attenuation in this case is in the region of 20 dB.
In the antenna arrangement shown in FIG. 1, it is crucial that the first and second antennas Ant1, Ant2 are very well insulated from one another. If the active antenna, in this case the second antenna Ant2, were to couple signals into the passive, terminated antenna, in this case the first antenna Ant1, then these signals would likewise enter the third signal path SP3 via the dual-band directional coupler DRK and corrupt the measurements by the measuring devices ME_forward, ME_reflected in the third signal path.